A condition known as "pumpoff" occurs in a well when the quantity of liquid being discharged by the pump per stroke is much less than the volumetric displacement of the pump. If the pump continues to operate without sufficient supply of liquid, the polished rod which transmits motion to the pump and its seals may overheat and be damaged or destroyed. Once pumpoff occurs, it is usually desirable to stop the pump to eliminate needless energy use and equipment wear. The pump may be restarted again after a sufficient time has passed to allow replenishment of the liquid column in the well annulus.
Several devices which control a pump power source in response to some property of the pumping system are commercially available. Each of the different control devices monitors either directly or indirectly the quantity of liquid being pumped and disconnects the pump power source whenever the liquid being pumped falls below a predetermined value.
One group of control devices uses flow sensors which incorporate impellers, paddles, or differential pressure devices to measure the liquid flow rate from the pump at the wellhead. Mechanical devices like these tend to fail in service because of corrosion, scale build up and blockage by debris.
Beam monitor and rod tension devices are also used. Strain gauges are attached to the pump jack beam and mechanical or electronic tension gauges known as "dynomometers" are attached to the pumping rod or its support members to determine the stress on the rod-string as a function of time. The shape of the stress versus time graph is related to the quantity of liquid lifted by the down hole pump.
Some of the control systems use sonic sounding devices. An acoustical impulse is propagated down the annular region between the tubing and well casing. The return echo is analyzed to determine the height of the liquid in the annulus. The height and rate of change of height of the liquid column is used to determine a suitable pumping cycle.
Other control units use temperature sensors to determine the temperature of the polished rod. When the flow of liquid from the wellhead begins to decrease, the polished rod is no longer adequately cooled and its temperature increases because of frictional heating. The rising temperature of the polished rod is detected and used as a control parameter.
Temperature-based oil well fluid flow detectors are known, such as in the form of a flow tube having an upstream and a downstream distributed temperature sensitive element placed around the tube, and further having a distributed heating element placed around the tube downstream of the downstream temperature sensitive element as described in U.S. Pat. No. 3,438,254.
U.S. Pat. No. 2,707,440 describes an oil well pump control system for use in producing a well fluid at a temperature above atmospheric temperature which shuts off the pump once the temperature of the fluid falls below a certain threshold.
There is also an apparatus for sensing fluid flow or the lack of the same in a conduit, such as crude oil in a pipe, described in U.S. Pat. No. 3,570,310. The apparatus comprises a fluid flow conduit made of a heat conducting material such as brass, an electrical heater mounted in thermal contact on the outside of the conduit, and an electrical switch responsive to changes in temperature mounted on the conduit, whereby an increase in the temperature of the conduit causes the electrical switch to trip. The switch is connected to an indicating and recording device such as a chart which shows the amount of time that it is switched to the higher temperature level and vice versa. In operation, the electrical heater heats the metal conduit and the fluid flowing through the conduit. If no flow occurs in the conduit, the conduit rapidly heats up to a higher temperature level and causes the temperature sensing switch to trip, remotely indicating to the recording device that the switch is in the no-flow position.
Some oil well control units determine the rate at which electrical energy is consumed by the pump power source since the amount of power used depends on the quantity of liquid being pumped.
Of interest is a device described in U.S. Pat. No. 4,311,047 to Charlie J. Hubbard, Jr., et al. That patent describes a fluid presence detector which detects the presence of liquid which is used to control pumping of a well when the quantity of liquid being discharged by the pump per stroke is much less than the volumetric displacement of the pump. The fluid detector includes a first thermally conducting body, a first heater, a first temperature detector, insulation material for thermally insulating the first thermally conducting body, a separate, second thermally conducting body physically exposed to the inside of a conduit, a second heater for heating the second thermally conducting body and a second temperature detector for detecting the temperature of the second thermally conducting body or the second heater. The two thermally conducting bodies are thermally insulated from one another, and the same insulation surrounds both. One arrangement also includes an electrical bridge network for comparing the relative rates of heat loss from the two thermally conducting bodies.
One disadvantage with the configuration of the '047 patent device is that the detector would be subject to corrosive attack. Thus, it would be an advance in the art if the detector could be made more resistant to corrosion. Further, the '047 device had trouble with temperature drift over the course of its use.